Suppose that, while browsing the web, you get to a page that has
multiple links and it is not clear which one (if any) will lead to
the information you're looking for. Maybe the desired information is
just one or two clicks away, in which case it makes sense to click on
a link, and if you don't ﬁnd you're looking for, click the back
button and try the next link. If the information is more than a few
clicks away, it might be better to open the link in a new tab in
which you can explore it to arbitrary depths. That way, if you
eventually decide that wasn't the way to go, you can close the tab,
and easily try a different path. Another option is to open each link
in its own tab, and explore all of them "concurrently".

Something like the tabs of a web browser would be even more useful in
a programming language, where undoing actions is a lot trickier than
clicking a back button. As an example, consider the task of
programming a robot to open a locked safe, as shown in Figure
1. There are two keys, A and B (each in its own room), but only one
of them unlocks the safe. Using a conventional programming language,
we might tell the robot to grab key A from room A, then go to the
safe and try to unlock it. At this point, if we ﬁnd that key A does
not open the safe, we probably want to have the robot clean up after
himself before trying the next alternative (nobody likes a messy
robot). So we must tell the robot to take key A back to room A, and
then return to its initial position.

In a programming language that supports "tabs", these clean-up
actions would not be required: we could simply open a new tab, and
inside it try to open the safe with key A. If A turns out to be the
wrong key, we can simply close this new tab to return to the initial
conditions.

This paper explores the idea of "tabs for programming languages",
which we call worlds.

I like this idea. I like that they used Javascript to build their
prototype even more. Javascript's extensibility and flexibility make
it an interesting place to explore new programming constructs.